The present invention relates to a lithographic printing plate capable of making a plate by an electrophotographic system. More particularly, the present invention relates to a lithographic printing plate comprising a negative charge type photosensitive material having sensitivity to white light or a short wavelength light source such as semiconductor laser, LED array or the like, and a polyester base subjected to a treatment for imparting electrical conductivity.
As a high-sensitivity lithographic printing plate utilizing an electrophotographic system, a zinc oxide master paper obtained by coating a zinc oxide-resin dispersion photosensitive material on a paper base subjected to a process for imparting water and solvent resistance has hitherto been used. The master paper has recently been classified into two types for analogue plate making and digital plate making which, respectively, use different photosensitive materials, due to the different exposure systems for plate making. The analogue plate making is performed by a conventional camera exposure system, which is a system of exposing a zinc oxide photosensitive layer to reflected light, obtained by irradiating an original with light, through lens or mirror to form an electrostatic latent image. On the other hand, the digital plate making is a system of sending image data made by using a computer to a light source such as gas laser, semiconductor laser, LED array or the like, as a digital signal, and directly exposing a zinc oxide photosensitive material to a signal of light obtained from the light source to form an electrostatic latent image. In both systems, a lithographic printing plate is obtained by developing with a developer. Using these two types of master papers, a plate product is obtained through an electrophotographic process, however, it is necessary that a non-image area of a zinc oxide-resin dispersion other than a toner image of the plate product is subject to a hydrophilization treatment in order to be used as a lithographic printing plate. A hydrophilizing solution is, for example, an aqueous solution containing a ferrocyanide salt or phytic acid as a main component (so-called etching solution). Since zinc oxide and the above material forms a hydrophilic chelate compound on the surface, the surface physical properties such as hydrophilicity and hydrophobicity required for lithographic printing, are formed on the surface, thereby making it possible to perform printing. This process is referred to as a direct plate making and has advantages such as a reduced plate making time due to simplicity of the plate making step and low running cost.
However, in case of this master paper, an irregularity of the surface of fibers of the paper and coarse grains of zinc oxide could cause deterioration of the image quality in plate making and printing. At the time of offset printing, the paper is liable to be influenced by water and problems such as expansion, wrinkling and the like are liable to arise. On the other hand, an improvement in a paper base quality by means of an aluminum laminate, polyethylene laminate or the like and an improvement in the smoothness of the zinc oxide photosensitive material have been studied to solve the problems associated with the base and the zinc oxide photosensitive material. However, the resulting master papers have both merits and demerits and satisfactory master papers have not be obtained heretofore.
As a general lithographic printing plate, a PS plate (PRE-SENSITIZED PLATE) obtained by coating a positive photosensitive material of a diazo compound on an aluminum plate as a base which utilizes photodegradation and a PS plate obtained by coating a negative photosensitive material of an acrylic prepolymer etc., which utilizes photosetting are put into practical use. These plates are obtained by a system of making an original from a silver salt film, performing adhesion exposure using an ultraviolet lamp and subjecting the non-image area to an alkali eluation treatment to obtain a printing plate. Since the base is an aluminum plate, the smoothness of the surface, the mechanical strength and the water resistance are sufficient for offset printing. On the other hand, there is a problem in that the plate making step is complicated and a long plate making time is required, resulting in a very high cost. Recently, a digital exposure system for the PS plate has been suggested and a system capable of making a plate using an argon laser or a helium-neon laser has recently been put into practical use. However, the system was not satisfactory because of problems of stability with time, the cost of the plate material and the like.
There has also been suggested an OPC plate material wherein the base is an aluminum plate and the photosensitive material is an electrophotosensitive material. The sensitivity of this plate material is sufficiently fast but the cost of the plate material is high because the base is an aluminum plate.
An object of the present invention is to provide an electrophotographic printing plate, comprising a photosensitive material and a base, which has high sensitivity to various light sources such as a halogen lamp, an argon laser, a helium-neon laser, a semiconductor laser, a light emitting diode and the like, high surface smoothness, high water resistance and excellent printability.